Date: July 20, 2011

Title: Encore: The Case of the Lobate Scarps

Podcaster: Carolyn Collins Petersen

Organization: Loch Ness Productions (

Links: Music by Geodesium (

Description: Meet shadowy planetary science detective Caloris Basin as she outlines the case of Mercury’s lobate scarps. This tale of planetary science intrigue began with the Mariner mission and continues with the Mercury MESSENGER mission.

This podcast originally aired on March 25, 2009.

Bio: Carolyn Collins Petersen is a science writer and show producer for Loch Ness Productions, a company that creates astronomy documentaries and other materials. She works with planetariums, science centers, and observatories on products that explain astronomy and space science to the public. Her most recent projects were the Griffith Observatory astronomy exhibits in Los Angeles and the California’s Altered State climate change exhibits for San Francisco’s California Academy of Sciences. She has co-authored several astronomy books, written many astronomy articles, and is currently working on a new documentary show for fulldome theaters, a vodcast series for MIT’s Haystack Observatory, and a podcast series for the Astronomical Society of the Pacific.

Sponsor: This episode of the “365 Days of Astronomy” Podcast is sponsored by Shane Miles of Safety Bay in Western Australia and is dedicated to all astronomers, past, present and future. Thank you for making my life richer. Thank you.


The Case of the Lobate Scarps. November 25, 2009

Hi there. Welcome to the Case of the Lobate Scarps. The name’s Basin, Caloris Basin – and I’m here to tell you the facts about the mysterious lobate scarps on Mercury. I’m a detective — a private eye with a penchant for planets – that is, when I’m not tracking down derelict bank executives and phony two-bit politicians. That’s what I do to make money. In my spare time, I’m puzzled by mysteries in planetary science. I got hooked on ‘em in grad school when I was studying forensic science – and astronomy was my second love. I became intrigued by the appearance of sulfur dioxide plumes in the Venus atmosphere. After that, it was dust devils on Mars. Plenty of places in the solar system with secrets to uncover, you know what I mean?

Look, I read the papers, I listen to the talks, and I’ve even been known to make a few suggestions to scientists now and again. Like what, you ask? Well, let’s just say that if the IAU had listened to me, that business with Pluto would have turned out a whole lot better.

So, what’s my latest interest? The planet Mercury. Not long ago, I learned the solution to a mystery about Mercury that’s bugged me since grad school.

It happened on a Friday afternoon a few weeks ago. I was planning to catch up on the journals that had piled up while I was out investigating some half-baked con man who claimed to heal the sick by laying his hands on their heads. Turns out he was laying his hands on their wallets. I caught him in the act and now he’s headed for Sing-Sing. That’s where he’s gonna be doing a little planetary science of his own — making little rocks outta big ones.

Anyway, things were quiet and I was looking forward to a nice weekend with my stack of back issues of Icarus.

First, I fed my boa constrictor — his name’s Twisted Sister and he’s been with me since grad school. Then, I pulled out a six-pack of Red Bull and two bags of Cheetos and settled back for a long read.

I was just digging in to a tasty article called “Fugitives from the Vesta Family” (Nesvorny, et al., volume 193, January 2008, pages 85-95) when my computer pinged me.

Hey, whaddaya know – there was an update about my favorite planet — Mercury.

It was a Google News alert about the MESSENGER spacecraft orbiting the planet. Had it really been five years since it launched? Huh! Well, I clicked through to the Web site and read the accumulated press releases.

Just looking at the accompanying images brought back memories of college and my long-time fascination with Mercury’s mysterious lobate scarps.

Back then, those scarps (cliffs to you regular joes) were evidence that something had happened to Mercury. But, what? All we had at the time were the old Mariner 10 images, and they didn’t have quite enough resolution to reveal everything about how those cliffs might have formed. We all wondered, what caused those cliffs and cracks? Volcanism? Big impacts that might have cracked the surface? And, when did they form? Those were key questions behind the mystery of the scarps. Questions we debated for hours in our crowded, 10-person grad student office over countless pizzas and cans of Mountain Dew.

My computer beeped again, jerking me out of my former-grad-student reverie and back to the present. I clicked through to the MESSENGER image gallery. There were pictures there of most of the ENTIRE planet — scenes of the “far side” of Mercury that we never even see from Earth because of the planet’s complicated spin-orbit resonance with the Sun. I wanted to crawl right through that monitor and walk on the surface of Mercury — the images were THAT good!

Okay…. there were the usual craters (old AND new), but what caught my attention were the scarps. Ah, there were the ones I studied in the Mariner images.

And, there were even MORE scarps, including one huge wrinkle that appeared to be one of the largest ever found on Mercury. In another place, old lobate scarps seemed to be cutting across even older craters. Classic evidence you’d see at any surface evolution scene. I could hardly contain myself. And, it was only the beginning of MESSENGER’s data dump.

Well, I quickly set to work making notes, studying each picture and thinking about what it added to the story of Mercury’s surface evolution. We know Mercury began like all the rocky planets, hot and molten. While Earth, Venus, and Mars basked in relative coolness out away from the Sun, Mercury stayed hot for quite a while. But, then it cooled, and as it did, its surface was blasted with impacts, digging out those craters. But, what about those huge, jagged lobate scarps? The nagging question remained: where did they come from?

Well, here’s the best theory based on the evidence we’re seein’ now. As the infant planet Mercury cooled, it shrank. The shrinkage compressed and wrinkled the rocky surface – creating what geologists call thrust faults. They look a lot like cliffs – and THOSE are the mysterious lobate scarps.

Now, I don’t pretend to know the whole cooling and bombardment history of Mercury. That’s a story planetary scientists are still working on. They’re examining each picture and each region on the planet to deduce the surface history. It’s pretty standard planetary science detective work.

But, they’re up to it — and so am I. As long as MESSENGER sends pictures, I’ll be following the case of the lobate scarps. And, now that we have images of Mercury’s polar regions, there’ll be a new challenge: finding out if ice might be hidden in the frigid craters at the poles.

Planetary science detective work: I love it!

Hey – you want to know more about Mercury and the MESSENGER mission and those mysterious lobate scarps? Point your browser to and click on the 365 Days of Astronomy tab. And, hey — thanks for dropping by.

End of podcast:

365 Days of Astronomy
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at or email us at Until tomorrow…goodbye.